1. Field of the Invention
The present invention relates to a differential control system for use in a 4-wheel drive vehicle.
2. Description of the Related Art
Front wheel drive vehicles exhibit more stability than rear wheel drive vehicles when the vehicles travel straight ahead. However, front wheel drive vehicles provide poor cornering because the tires must be maintained at a large angle due to the loss of front wheel cornering power. The rear wheel drive vehicles exhibit excellent cornering. However, they are susceptible to oversteering during cornering.
Thus, the ideal running of a vehicle is with front and rear axles driven be an equal force. From this viewpoint, 4-wheel drive vehicles are excellent because the engine torque is continuously distributed between front and rear axles by means of a central differential device.
In the 4-wheel drive vehicle, the engine torque is first transferred to the central differential device which is a gear unit including a pinion gear and right and left side gears or a planetary gear unit. The central differential device splits the engine torque into two fractions. One of the torque fractions is transferred from the central differential device to the front axle differential device where it is distributed again to the right and left side gears. The distributed torques are then transferred to the right and left front axles. The other fraction is transferred through a propeller shaft to a rear axle differential device where it is distributed again to the right and left side gears. The distributed torques are then transferred to the right and left rear axles.
When the engine torque is split between front and rear differentials by the central differential device, the so-called tight corner braking phenomenon is prevented, which would otherwise occur due to different angular rates for each wheel during tight cornering on a road having a high frictional coefficient, while the torque transfer limit values during cornering increase. Maximum traction occurs when equal torque drives the front and rear wheels.
However, in the 4-wheel drive vehicle which incorporates a central differential device, when one of the four wheels slips on a snowy or frozen road and no load is applied thereto, drive energy escapes through that one wheel by the operation of the central differential device, causing power loss.
Hence, a differential limiting mechanism for suspending the operation of the central differential device is provided. The differential limiting mechanism includes a wet multiple disk friction clutch and a hydraulic actuator.
In a normal running, the multiple disk friction clutch is disengaged, and the central differential device is maintained in an operational state to split the torque between front and rear axles. When the vehicle travels on a road which is covered with snow or on a frozen road, the clutch is engaged to suspend the operation of the central differential device.
The slip limit characteristics of the tires are determined by a load applied perpendicular to the surface of the tire at the point where it makes contact with the ground, and the slip limit characteristics vary in accordance with distribution of the weight of the vehicle. However, drive train wind-up develops in rigid 4-wheel drive transmissions with the above-described conventional clutch locked-up central differential device, causing power loss in the torque transmission system and increased wear of the clutch and gear components.
In the clutch open mode, the center differential is initially efficient in the transfer of torque; however, as torque further increases, either the front or rear wheels slip first, preventing further transfer of the torque. In other words, the torque transfer limit is determined by the lower torque value of the front or rear wheels. For example, one of the front wheels slips, the torque of the rear wheels also reduces immediately, and this prevents sufficient driving force from being transferred to the road surface.
In answer to the foregoing problem, Japanese Patent Laid-Open No. 176728/1988 discloses a system in which a differential limiting clutch is provided parallel to the central differential device and in which engagement of the differential limiting clutch is limited by an electronic control unit so as to achieve increase in the slip limiting characteristics of the tires and increase in climbing ability, traction and running stability on a snowy road or the like. Such a differential system that continuously varies the torque distribution responsive to traction requirements and optimal fuel economy would improve vehicle performance and offer more safety to the driver. However, the differential limiting clutch employed in the above-described differential control system suffers from problems associated with wear and, after an extended period, fails to provide sufficient differential limiting.